I don’t want smartwatches (they require a specific app from a specific developer to sync)
Look into GadgetBridge. It’s an app to connect to smart watches that’s so privacy friendly it forbids any Internet access at all. A Pebble watch will have week long battery life, which is pretty good, and any other solution is going to need to be charged at least that much.
I was exactly going to mention that. Most compatible devices on the list are either out of stock or too expensive from my location. Now, the list also mentions a chinese brand/manufacturer(?) called ‘XWatch’ that looks interesting, but the name’s too generic to get anything specific in aliexpress.